The invention relates to a method of removing sodium salts from mixtures of polyphosphazene polymers and such salts. More particularly, the invention relates to an improved method of removing sodium salts from mixtures of polyphosphazene polymer solutions and such salts which features the use of a sulfated fatty acid ester of a natural nonpetroleum based oil as a surfactant in effecting a more complete separation of the mixture into distinct polyphosphazene polymer solution and brine phases thereby allowing for more complete removal of the sodium salts from the polymer solution.
The method of the invention is applicable to the removal of sodium salts from mixtures of said salts and solutions of polyphosphazene polymers represented by the formula: ##STR1## wherein R and R' may be the same or different and are selected from the group consisting of alkyl, substituted alkyl, aryl, and substituted aryl groups and n is from 20 to 50,000. The substituted or unsubstituted alkyl and aryl groups may be any of those heretofore known in the polyphosphazene art which may be incorporated into the polyphosphazene polymer by the so-called alkoxide or aryloxide derivatization reaction. Thus, such substituents may be any of those described in such U.S. Pat. Nos. as 3,370,020; 3,515,688; 3,700,629; 3,702,833; 3,856,712; 3,856,713; 3,853,794; 3,883,451; 3,970,533; and 4,225,697, the disclosures of which are incorporated herein by reference. Typical substituted alkyl groups which may be present in the polymers include fluorine substituted alkyl groups such as those described in the aforementioned incorporated patents. Typical substituted aryl groups which may be present in the polymers include alkyphenyls, halogen-substituted phenyls and alkoxy-substituted phenyls such as those described in the aforementioned patents.
Mixtures of sodium salts and solutions of polyphosphazene polymers of the above formula are generally prepared in well-known manner by reacting in solution polydichlorophosphazene polymers --NPCl.sub.2).sub.n in which n is from 20 to 50,000 with an alkoxide or aryloxide represented by the formula NaOR or NaOR' wherein R and R' are defined above.
Prior methods of removing sodium salts from such mixtures and recovering the polymer from solution generally involve a process in which the mixtures are first neutralized with an acid such as sulfuric acid or hydrochloride acid to neutralize any excess aryloxide or alkoxide. Then, water is added to the mixture to extract the sodium salts (e.g., sodium chloride, sodium sulfate or mixtures thereof) from the mixture to form two separate phases; an upper polyphosphazene polymer solution phase and a lower brine phase consisting of water and sodium salts. The brine phase is then separated from the polymer solution phase, usually by draining off the lower brine phase. The polyphosphazene polymer is then generally recovered from solution by pumping the remaining polymer solution through a coalescer, preconcentrating the polymer solution with an alkane (e.g., hexane), steam desolventising the polymer solution to remove organic solvents and then drying the polymer in a vacuum oven.
However, in such prior processes, a major problem often occurs in the water extraction process. Thus, the water extraction process often results in the formation of a partial or total emulsion. As a consequence, significant amounts of sodium salts are often entrapped in the polyphosphazene polymer solution and consequently in the dried polyphosphazene polymer. The presence of significant amounts of salt in the polyphosphazene polymer adversely affects the physical properties of the polymer. Thus, it is desirable to produce polymers having less than 1% salt and preferably 0.25% or less. Accordingly, when an emulsion forms during the water extraction step and excessive amounts of salt are entrapped in the polymer, it is necessary to subject the polymer to additional processing to remove the excess salt. This generally involves wash milling the polymer, a process in which the polymer is milled and washed with water for time periods of up to 30 minutes. In order to achieve the desired low levels of salt, it may be necessary to conduct multiple wash milling procedures. As will be evident, such procedures are both time consuming and labor intensive.
The prior art has alluded to the formation of emulsions during the water extraction process and suggested several proposed solutions to the problem. Thus, for example, U.S. Pat. No. 3,985,835 to Kao suggests that as a precaution against the formation of emulsions, the water should be maintained at a pH of 9 or higher. The reference further teaches that should an emulsion appear during the water extraction procedure, sodium chloride or other salt should be added. Additionally, the article entitled Fluoroalkyl Phosphonitrilates, A New Class of Potential Fire-Resistant Hydraulic Fluids and Lubricants, by Henry Lederle, et al., Volume 11, No. 2, Journal of Chemical & Engineering Data discloses the formation of an emulsion during water-washing and the use of sodium sulfate to break the emulsion. However, applicants have found that the use of sodium salts to prevent the formation of or to break emulsions formed during the water extraction process has been singularly ineffective.
In view of the foregoing, the provision of a simplified process for handling emulsions which form during the water extraction process and for promoting a more complete separation of the polymer solution phase from the brine phase would constitute a highly significant development.